1. Field of the Invention
The present invention relates to an electromagnetic wave focusing device, and more particularly to an electromagnetic wave focusing device which can be used as a sunlight focusing device in a spacecraft propulsion system that is utilized in order to heat propellant molecules directly in the form of thermal energy without any conversion of solar energy into electric power or the like, or which can be used as a transmitting and receiving device of electromagnetic wave used in communications.
2. Description of the Related Art
A spacecraft propulsion system utilizing solar heat has not yet been realized. However, solar thermal propulsion systems can utilize hydrogen, which is the lightest molecule, as a propellant; accordingly, such propulsion systems make it possible to realize a high specific impulse that is approximately 2 to 4 times that of conventional chemical propulsion (800 to 900 seconds). In terms of specific impulse, this does not reach the level of ion propulsion. In the case of ion propulsion, however, there is a loss due to the fact that the energy of sunlight undergoes two conversions, i.e., a xe2x80x9clight-electric powerxe2x80x9d conversion performed by a solar battery, and an xe2x80x9celectric power-electric powerxe2x80x9d conversion performed by a power processor. Accordingly, solar thermal propulsion is superior in terms of energy per unit light-receiving area, so that if the light focusing apparatus can be made sufficiently light-weight, solar thermal propulsion which utilizes solar energy directly is more advantageous.
It is conceivable that a rotational-parabolic mirror or a portion of such a mirror might be utilized as a sunlight focusing device for focusing sunlight in one place in a spacecraft propulsion system utilizing solar heat. In addition to light-weight, high precision may be cited as one of the conditions required in a mirror that is utilized in a sunlight focusing device. However, these two requirements are in a conflicting relationship. Specifically, the mirror surface body must be made thin in order to reduce the weight, but if this mirror surface is made thin, a high degree of precision cannot be maintained in the case of a self-standing mirror, and in the case of a system in which the mirror surface is installed along a frame assembly, the mass of the frame assembly is added, so that it becomes difficult to reduce the weight. If such points are taken into consideration, the limits have more or less been reached in any approach that utilizes a conventional focal-point rotational-parabolic surface.
As described above, in the case of mirrors that utilize a conventional rotational-parabolic surface, satisfactory performance as a spacecraft propulsion system could not be obtained with the combination of mass and precision that could be achieved. Furthermore, in cases where a spacecraft propulsion system is carried into space, the system must be folded up to be loaded inside the fairing of the launch rocket; as a result, partitioning the mirror leads to a much greater increase in mass and drop in precision.
With the foregoing in view, it is an object of the present invention to provide an electromagnetic wave focusing device, or more concretely a sunlight focusing device, which makes it possible to realize a lighter weight with the same light focusing performance, or to realize a higher light focusing performance at the same mass per unit light-receiving area, compared to a sunlight focusing device with a mirror that utilizes a conventional rotational-parabolic surface.
The present inventor conducted various studies based on the following idea: namely, it is theoretically possible to cause the complete convergence of plane waves at one point by combining two perpendicular focal lines, or more specifically, by combining parabolic cylinder mirrors so that the mirror image of the focal line of a primary reflective mirror in a secondary reflective mirror and the focal line of the secondary reflective mirror are perpendicular, as a method that can be used in place of a method that utilizes a rotational-parabolic surface in a sunlight focusing device. The formation of a parabolic surface that has a focal line is far easier than the formation of a rotational-parabolic surface. Ordinarily, a surface that is close to a parabolic surface with a focal line can easily be formed; however, the formation of a rotational-parabolic surface is extremely difficult. For example, in the case of a film which possesses elasticity such as a rubber film, a rotational-parabolic surface can be approximated to some extent; however, a large force is necessary in order to maintain the parabolic surface. The fact that a surface that is close to a parabolic surface with a focal line can be easily formed is useful in constructing a light-weight, high-precision mirror, and in the case of a parabolic surface with a focal line, the mirror itself can be made extremely thin if a frame assembly is installed at two ends and the aperture; furthermore, the frame assembly itself can also be simplified by further expedients.
The electromagnetic wave focusing device of the present invention is based on the above findings; this electromagnetic wave focusing device comprises a mirror group consisting of a focal-line type primary reflective mirror and secondary reflective mirror that have a unique parabolic cross-sectional shape in an arbitrary cross section that is perpendicular to a certain direction; furthermore, this electromagnetic wave focusing device is devised so that incident plane waves are primarily reflected by said primary reflective mirror, and are then secondarily reflected by said secondary reflective mirror, thus causing these plane waves to be focused at a single point. As described above, the electromagnetic wave focusing device of the present invention is not limited to a sunlight focusing device, but can also be utilized as a transmitting and receiving device of electromagnetic wave used in communications.